This invention relates to solid oxide fuel cells having a ceramic trilayer construction and, more particularly, to the sintering of the final structure.
A fuel cell is an electrochemical conversion device that chemically reacts a fuel, such as hydrogen or a hydrocarbon, with an oxidizer in an elevated temperature, catalytic environment. The hydrogen reacts with the oxygen to produce carbon dioxide and water, and an electrical current through an external circuit to provide power. The fuel cell provides a structure having internal passageways that permit the fuel and oxidizer to interdiffuse at a controlled rate to produce this reaction.
Several designs of fuel cells have been utilized in the past. In one popular approach, a support tube provides the structural support for electrochemically active layers of cathode, electrolyte, and anode materials deposited on the exterior surface of the support tube. The oxidant flows on the interior of the tube, and the fuel flows on the exterior of the tube. The reaction between the two in the active layers produces the electrical current. A typical fuel cell of this type would utilize a number of such elements operating in series to obtain higher voltages or in parallel to obtain higher currents.
Such tubular designs have structural and operating limitations, and a number of modifications have been made to improve their performance. Dimensions and compositions have been optimized, the elements have been arranged in various configurations, and the active layers have been interconnected in different ways between elements. However, there remained fundamental limits to fuel cell performance.
As a result, other fuel cell designs have been developed. In one alternative approach, the active materials are formed as slurries and painted onto a surface. This approach reduces the weight and improves the ability to control the thickness and location of the active materials.
In another approach, a fuel cell is fabricated using a monolithic structure wherein the separate support structure is largely eliminated. In this design, the active materials themselves provide the structural strength required for integrity of the fuel cell. In one successful design, described in U.S. Pat. No. 4,816,036, the powders of the ceramic materials utilized in active anode, cathode, electrolyte, and interconnects are separately mixed with a binder and a plasticizer to form a slurry, each of which is then processed into a tape form. The tapes are combined in trilayer form as trilayer electrolyte walls and trilayer interconnect walls. The trilayer walls in turn are assembled to form a fuel cell element, which is sintered to form a monolithic structure. In this monolithic fuel cell, the sintered trilayers provide not only the electrochemically active material but also the structure for the fuel cell.
The trilayer monolithic fuel cell approach has been successful. However, it has been observed that, although the electrical and mechanical properties of the fuel cell are good, in theory they might be made even better. That is, it may be possible to increase both the electrical properties, in the form of electrical conductivity and output current density, and mechanical properties, in the form of fuel cell structural strength.
Thus, there is always an ongoing need for improving the characteristics of such fuel cells. The present invention fulfills this need, and further provides related advantages.